A sign (felt-tip pen on foam board), reading Bed Peace, is expected to fetch up to £100,000 ($160,000). Photograph: Christie's

A handwritten sign John Lennon and Yoko Ono displayed during one of their famous "bed-ins" has emerged for sale.

The sign (felt-tip pen on foam board), reading Bed Peace, is expected to fetch up to £100,000 ($160,000) at auction.

Lennon and Ono's seven-day "bed-in for peace" was held in 1969 at the Queen Elizabeth hotel in Montreal.

It was hung above their bed alongside a second sign reading Hair Peace. At the end of the event, it was taken by a member of a film crew, who passed it on to a colleague.

The notice is signed and dated by Lennon and Ono, and features a caricature of both of them drawn by the former Beatle.

Neil Roberts, of Christie's, said it was the first time the public had seen the sign for 42 years.

Roberts said: "This sign is emblematic of that period of time and the event. The wording was outlined and then coloured in with black felt tip. It really is an iconic item, and collectors would love to own it.

"Both John and Yoko have signed and dated it, and John has added a caricature of them both. John and Yoko produced many artworks during the event but this was kept prominently above the bed.

"It was taken by a member of a film crew who gave it to a friend, and it has been in that Canadian family ever since."

The Montreal bed-in came two months after the couple's honeymoon bed-in in Amsterdam, their take on a sit-in. A central idea, as espoused by Lennon, was to stay in bed and grow your hair rather than doing anything violent.

It began at the end of May 1969 and lasted a week. The couple opened their door to the world's media and spoke to journalists, politicians and artists trying to promote the idea of peace in the world. They also found time to record the anthem Give Peace a Chance.

Many artworks and placards were created over the week and were moved around and given away but the Bed Peace sign was a constant.

It was taken by a freelance sound man who was in the room as the event drew to a close.